In electrophotography an image comprising an electrostatic field pattern, usually of non-uniform strength (also referred to as an electrostatic latent image), is formed on an insulative surface of an electrophotographic element comprising at least a photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the insulative surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise radiation-induced dissipation of the initially uniform electrostatic field is brought about by the creation of electron/hole pairs, which are generated by a material, often referred to as a photoconductive or charge-generation material, in the electrophotographicc element in response to exposure to imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic field and the types of materials included in the electrophotographic element, part of the charge that has been generated, i.e., either the holes or the electrons, migrates toward the charged insulative surface of the element in the exposed areas and thereby causes the imagewise dissipation of the initial field. What remains is a non-uniform field constituting the electrostatic latent image.
Several types of electrophotographic recording elements are known for use in electrophotography. In many conventional elements, the active photoconductive materials are contained in a single layer. This layer is coated on a suitable electrically conductive support or on a non-conductive support that is overcoated with an electrically conductive layer. In addition to single-active-layer electrophotographic recording elements, various multi-active electrophotographic recording elements are known. Such elements are sometimes called multi-layer or multi-active-layer elements because they contain at least two active layers that interact to form an electrostatic latent image.
A class of photoconductive materials useful in the aforementioned single-active-layer and multiactive elements is the class of perylene pigments, particularly perylene-3,4,9,10-tetracarboxylic acid imide derivatives. Such perylene photoconductive materials are often capable of providing exceptional performance in such elements. For example, U.S. Pat. No. 4,578,334, issued Mar. 25, 1986, describes multi-active electrophotographic recording elements that contain, as photoconductive materials, certain crystalline forms of N,N'-bis(2-phenethyl)perylene-3,4:9,10-bis(dicarboximide) characterized by particular spectral absorption and x-ray diffraction characteristics. Also, U.S. Pat. No. 4,714,666, issued Dec. 22, 1987, describes single-active-layer electrophotographic elements and multi-active elements containing, as photoconductive materials, asymmetrically substituted perylene-3,4,9,10-tetracarboxylic acid imide derivatives. In addition, U.S. Pat. No. 4,719,163, issued Jan. 12, 1988, describes multi-active electrophotographic elements that contain, as photoconductive materials, the compound N,N'-bis[2-(3-methylphenyl)ethyl]perylene-3,4:9,10-bis(dicarboximide).
Unfortunately, electrophotographic recording elements of the prior art have typically suffered from one or more disadvantages that have significantly restricted their use. For example, vacuum sublimation is frequently required to deposit photoconductive perylene materials in a crystal form suitable for high speed electrophotographic elements. Vacuum sublimation, however, requires expensive equipment for production scale runs and thin sublimed films are fragile and susceptible to damage until they can be protected by a more durable overcoat. It is evident therefore, that electrophotographic recording elements comprising photoconductive perylene materials that provide high photosensitivity without requiring vacuum sublimation coating techniques would represent a significant advance in the art. It is an objective of this invention to provide such electrophotographic recording elements.